The present invention relates to the art of making epoxy resins.
Liquid epoxy resin is usually made by reacting a dihydric phenol, such as bisphenol A, with excess epihalohydrin in the presence of an alkali metal hydroxide, such as sodium hydroxide. Unused epihalohydrin and catalyst are usually recycled back into the reaction. See, for instance, Wang et al, U.S. Pat. No. 4,499,255 (Feb. 12, 1985) and Massingill, U.S. Pat. No. 4,313,886 (Feb. 2, 1982). The resulting resin usually contains predominantly the diglycidyl ether of the dihydric phenol, with minor quantities of oligomer and or resin that is terminated by .alpha.-glycol groups.
The dihydric phenol and the diglycidyl ether are preferably represented by Formula 1: ##STR1## wherein:
Ar represents an aromatic group.
Each Q is a hydroxyl group in the dihydric phenol, and Q is a glycidyl ether moiety represented by Formula ( 2 ): ##STR2## in the epoxy resin. Each R represents a hydrogen atom, a halogen or a lower alkyl group.
"n" represents a number of repeating units. "n" is usually 0.1 to 0.2 in liquid epoxy resin. PA0 (1) the reaction temperature is between 130.degree. C. and 200.degree. C.; PA0 (2) the reaction mixture contains between 0.5 and 10 parts water per 100 parts resin by weight; and PA0 (3) the mixture contains 0 to 1 weight percent organic solvents and 0 to 100 ppm each of: organic acids, organic diacids, phosphonium compounds, and alkali or alkaline earth metals and their salts.
Usually, a small percentage of the glycidyl ether moieties (Q) are hydrolyzed during this process to make .alpha.-glycol groups, which are preferably represented by Formula (3): ##STR3##
For many uses, it is desirable for the resin to contain only a very small amount of .alpha.-glycol. First, resins that contain more than about 2 percent .alpha.-glycol are difficult to wash after they are made because they form emulsions when impurities are washed from the resin using water. Second, resins that contain very low levels of .alpha.-glycol have superior performance properties for many applications, such as high glass-transition temperature in electrical laminates. Therefore, it is frequently desirable to manufacture liquid epoxy resin that contains very low levels of .alpha.-glycol. This can easily be done by removing hydrolysis products of epihalohydrin from recycled epihalohydrin before returning the epihalohydrin to the reaction vessel.
On the other hand, somewhat higher levels of mono-.alpha.-glycol-containing resin (in which one Q is a glycidyl ether moiety of Formula (2) and the other Q is an .alpha.-glycol of Formula (3)) are frequently desirable for several applications. Small amounts of mono-.alpha.-glycol resin greatly accelerate the curing of liquid epoxy resin with amine catalyst. Furthermore, resins that contain moderate amounts of mono-.alpha.-glycol resin interact better with thixotropic agents and flow modifiers. Therefore, it is desirable to increase the levels of mono-.alpha.-glycol resin in liquid epoxy resins that contain very little .alpha.-glycol, for some uses. However, bis-.alpha.-glycol resin (in which both Q are .alpha.-glycol moieties of Formula (3)) are undesirable and are preferably minimized.
It has been known in the prior art to make hydrolyzed epoxy resins that can be added to nonhydrolyzed resin to make a mixture containing desired amounts of .alpha.-glycol. For instance, Walker, U.S. Pat. No. 3,632,836 describes the hydrolysis of epoxy resins in a dilute aqueous solution with an acidic catalyst at a temperature of 50.degree. C. to 374.degree. C. Cavitt, U.S. Pat. No. 4,404,335 (Sep. 13, 1983) teaches to hydrolyze epoxy resins without an organic solvent using a catalyst that contains a dicarboxylic acid (such as oxalic acid) and a phosphonium compound.
The know processes have draw backs which make them undesirable for use in mass producing large quantities of liquid epoxy resin that contain moderate .alpha.-glycol content. First, they use catalysts and sometimes solvents that must be removed from the resin before it can be sold or used. Second, they make large quantities of bis-.alpha.-glycol species resin in addition to the mono-.alpha.-glycol resin.
What is needed is a process to moderately increase the level of mono-.alpha.-glycol resin in an epoxy resin to a desired level without the need for a catalyst or solvent.